FILE- In this July 9, 1960 handout file photo provided by the U.S. Navy, the nuclear-powered submarine USS Thresher is launched at the Portsmouth Naval Shipyard in Kittery, Maine. Fifty years ago 129 men lost their lives when the sub sank during deep-dive testing off Cape Cod. The deadliest submarine disaster in U.S. history delivered a blow to national pride during the Cold War and became the impetus for safety improvements. (AP Photo/U.S. Navy, file)

PORTSMOUTH — It was a radio transmission 50 years ago that first alerted Norman Bower to the trouble on board the USS Thresher.

It was April 10, 1963, and U.S. Navy ships had lost contact with the nuclear-powered submarine. Bower, who was serving as chief of the USS Albacore at the time, remembered hearing radio chatter about the Thresher’s disappearance.

His thoughts jumped to the fate of the crew, a group of 129 men that included three former shipmates. Among them was Wayne Lavoie, a father of five from Gonic.

Bower remembers it was another 16 or 20 hours before the Navy officially announced the ship had plummeted to the ocean floor, taking the crew members with it.

Now 77 and retired, Bower has worked over the years to help preserve the memory of the Thresher disaster. The tradition will continue this weekend, as Bower and hundreds of others gather on the Seacoast to reflect on the 50th anniversary of the ship’s sinking.

The Thresher catastrophe marked a turning point in the Navy’s safety operations. It also changed the trajectories of countless lives, including that of Peter Charron, of Rochester. Charron grew up never knowing his father, Robert Charron, a civilian sonar technician who perished on the ship. Peter Charron was 16 months old at the time.

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FILE- In this 1960 handout file photo provided by the U.S. Navy, the nuclear-powered submarine USS Thresher is prepared for launching at the Portsmouth Naval Shipyard in Kittery, Maine. Fifty years ago 129 men lost their lives when the sub sank during deep-dive testing off Cape Cod. The deadliest submarine disaster in U.S. history delivered a blow to national pride during the Cold War and became the impetus for safety improvements. (AP Photo/U.S. Navy, file)

Now 51, Charron said the circumstances of his father’s death propelled him toward his career in funeral services. As a high school sophomore, Charron decided to enter the field to help provide closure to other families in pain.

“I decided, ‘OK, I’m going to do for other people what dad couldn’t have,’” he said.

Charron will attend a memorial service at Portsmouth High School this afternoon with his 89-year-old mother, two sisters and two brothers. U.S. Submarine Veterans Incorporated Thresher Base, the group organizing the ceremony, anticipates that hundreds of surviving family members will be on hand.

The event is closed to the public, but the Defense Video & Imagery Distribution System is planning to broadcast the event live on the Internet. The ceremony, which starts at 1 p.m., is expected to be viewable online at http://www.dvidshub.net/webcast/2935.

A separate service will also take place in Kittery, Maine, on Sunday morning. The Thresher Memorial Project Group will dedicate a new 129-foot flagpole in the center of Memorial Circle to the victims. Ship’s bells will also ring every 10 seconds during the 9 a.m. ceremony.

Click image to enlarge

FILE- In this July 9, 1960 handout file photo provided by the U.S. Navy, the nuclear-powered submarine USS Thresher is launched at the Portsmouth Naval Shipyard in Kittery, Maine. Fifty years ago 129 men lost their lives when the sub sank during deep-dive testing off Cape Cod. The deadliest submarine disaster in U.S. history delivered a blow to national pride during the Cold War and became the impetus for safety improvements. (AP Photo/U.S. Navy, file)

The Thresher was the world’s most advanced fast attack submarine when it was commissioned in 1961. Built at Portsmouth Naval Shipyard in Kittery, and based in Groton, Conn., the ship featured a cigar-shaped hull and nuclear propulsion. The 278-foot-long submarine could travel underwater for long distances, and it could dive deeper than earlier submarines, enduring pressure at unforgiving depths. The Cold War-era vessel was also designed to be quieter to avoid detection.

The Thresher was conducting sea trials off the coast of New England with another ship when it sent a message at 9:13 a.m. stating, “We are experiencing minor difficulties, we have a positive up angle, and are attempting to blow. Will keep you informed.”

Only minutes later the other ship, the USS Skylark, received two garbled messages, followed by high energy, low frequency disturbance. The sound was the mangling of the ship, which had fallen below crush depth.

A special inquiry later determined the submarine likely sank due to a piping failure, loss of power and an inability to blow ballast tanks rapidly enough. An extensive search found the ship some 8,400 feet below the surface. The dead included 16 officers, 96 enlisted men, and 17 civilians.

For the families, the silver lining is that subs are now safer. The Navy accelerated safety improvements and created a program called “SUBSAFE,” an extensive series of design modifications, training and other improvements.

People involved in the SUBSAFE program are required to watch a documentary about the Thresher that ends with an actual underwater recording featuring the eerie sounds of metal creaking and bending as a U.S. Navy submarine breaks apart with the loss of all hands.

“It’s important because the real legacy of Thresher is the SUBSAFE program that the Navy initiated after the loss of Thresher, which has kept submarines and their crew safe since then,” said Lori Arsenault, of Gorham, Maine, whose father perished in the accident. “And the real important thing to understand 50 years later is the people making decisions are too young to remember that day, and so by consciously remembering it, we can keep working at making sure that it doesn’t happen again.”

Arsenault’s father, Tilmon J. Arsenault, was a World War II veteran who served as chief engineman on the Thresher. Lori Arsenault says her father was her mentor, and he got her interested in music and technology. She is now director of operations and concert manager for the School of Music at the University of Southern Maine.

“I sort of got my passion for these things from him,” she said.

Arsenault was 8 at the time of her father’s death. She recalls that her mother learned of the tragedy while attending a PTA meeting at the local school in the evening.

“The families — the Navy was trying to contact the families throughout the day, so it was even late at night and they were still trying to contact my mom. She was at a PTA meeting, and then they finally found her at the school. She came home — she doesn’t remember the drive home — but she came home and we all huddled in the kitchen and cried.”

Barbara Currier, whose husband, Paul, was a civilian worker on the Thresher, was shopping with her daughters when she heard the news on the radio in a store. What followed was a blur of activity for her family. Navy officers in dress whites showed up on doorsteps. Friends and neighbors brought food.

After the submarine was declared sunk, President John F. Kennedy ordered the nation’s flags lowered to half-staff. International leaders sent condolences.

“The men, they were heroes. Most of them were doing what they wanted to do for their country to keep the country safe,” said Currier, 86, who never remarried and still lives in the same house in Exeter, N.H. “They were pushing things to the limit.”

Because of their tender ages, and the lack of a body or proper grave site, children like Vivian Lindstrom, who lost her father, Samuel Dabruzzi, a Navy electronics technician, were unable to grieve properly.

Thanks to the reunions, they at least know they’re not alone, said Lindstrom, of Glenwood City, Wis.

“We’ve experienced the same things, felt the same things,” she said. “We feel like family. We call ourselves the Thresher family.”Foster’s staff writer Jim Haddadin and David Sharp of The Associated Press contributed to this report.